The article "Bipolar monolithic very-wideband amplifier" by T. Kamato, et al., published in the journal Electronics and Communications in Japan, Vol. 67-C, No. 4, 1984, pp. 85 to 92, describes a wideband equalizing amplifier comprising an input amplifier (buffer), an automatic gain control amplifier, an amplifier, and offset control circuit acting on the input of the input amplifier, and an automatic gain control circuit acting on the automatic gain control amplifier. That equalizing amplifier suffers from the drawback of having its gain control dependent on its offset control.
The article entitled "Wideband and high-gain negative-feedback AGC amplifier for high-speed lightwave digital transmission systems" by K. Yamashita et al., published in the journal Electronics Letters of May 9, 1985, Vol. 21, No. 10, pp. 419 and 420, describes an automatic gain control amplifier having the drawback of presenting a gain peak at high frequencies (at about 0.5 GHz) when operating at minimum gain, thus running the risk of oscillation; in addition, correcting this gain peak is difficult because it varies with gain.
An object of the invention is to make automatic gain control and offset control independent from each other.
Another object of the invention is to eliminate the risk of oscillation when the amplifier circuit is operating at high frequency and at minimum gain.
Proposals have also been made by the present Applicant in Document FR-2-642 918 for a wideband amplifier circuit essentially comprising two gain control amplifiers associated with a post-amplifier having a non-inverting output and an inverting output which are respectively connected to two peak detectors. A third amplifier whose output is connected to a gain control input of each of the first two gain control amplifiers has one input at a fixed potential and another input connected via two respective resistances to the above-mentioned two peak detectors.
In the present invention, the Applicant provides a novel type of wideband amplifier circuit which possesses a feedback loop for gain control and another loop for controlling and cancelling any offset voltage that may exist at its output.